1. Field of the Invention
This invention relates to a weld bonding method which combines use of an adhesive with resistance spot welding and, more particularly, to a weld bonding method which permits substantial improvement of the overall bonding strength of weld bonded structures by the use of adhesives. The present invention finds application in the bonding of various structures such as auto body panels.
2. Description of the Prior Art
In general, a weld bonding method includes the steps of bonding a plurality of metal sheets together by means of resistance spot welding (hereinafter referred to simply as "spot welding") through an adhesive, and subsequently heat-treating the bonded metal sheets to cure the adhesive, whereby both the welding and the adhesive contribute to the overall bonding strength.
For instance, in case of applying the weld bonding method to the bonding of auto body panels, the panels go through several steps after spot welding, and are finally subjected to a baking finish. In this case, the adhesive is also cured simultaneously with heating in the baking finish, and therefore, the adhesive and the welding are combined to obtain the compounded bonding strength. Incidentally, even before heat curing of the adhesive, since the panels go through several processes as described above, substantial bonding strength of a certain value due to spot welding is required. Further, even after heat curing of the adhesive or when an adhered portion is released for some reason, it is necessary that substantial bonding strength of a certain value should be ensured for a spot weldment.
However, according to the conventional weld bonding method, since the adhesive is between the plurality of metal sheets as described above, the flow of welding current is degraded to prevent generation of heat in spot welding, or resistance heat is absorbed in the adhesive with the result that a calorific value sufficient to melt the material to be welded may not be obtainable. Thus, the conventional weld bonding method involves problems in that melting may be insufficient, while the weld nugget is inferior in internal quality as compared to that obtained in general spot welding using no adhesive, and sufficient welding strength before heat curing of the adhesive is hardly obtained.
On the other hand, when the current value is raised so as to sufficiently form a nugget, the conventional weld bonding method involves problems in that an electrode material is deposited on the material being welded or an explosion may occur to hardly obtain sufficient welding strength.
Bonding with the adhesive is adapted to obtain high strength by curing the adhesive through heat treatment, so that the bonding strength itself before heat curing of the adhesive is low as a matter of course.
Incidentally, the related art, e.g. Japanese Application Laid-open Nos. 60-173075, 61-4780, 62-68874, 64-53780, 2-150485 and 2-255883 disclose a proposal in which various kinds of metal fibers or the like are added to an adhesive so as to enhance weld bonding strength. However, even using this approach, it has been difficult to obtain sufficient welding strength before heat curing of the adhesive.